The term OPS-lasers, as used herein, refers to a class of vertical-cavity surface-emitting semiconductor lasers wherein optical gain is provided by recombination of electrical carriers in very thin layers, for example, about 15 nanometers (nm) or less, of a semiconductor material. These layers are generally termed quantum-well (QW) layers or active layers.
In an OPS-laser, several QW layers, for example, about fifteen, are spaced apart by separator layers also of a semiconductor material, but having a higher conduction band energy that the QW layers. This combination of active layers and separator layers may be defined as the gain-structure of the OPS-laser. The layers of the gain-structure are epitaxially grown. On the gain-structure is an epitaxially-grown multilayer mirror-structure, often referred to as a Bragg mirror. The combination of mirror-structure and gain-structure is referred to hereinafter as an OPS-structure or an OPS-chip.
In an (external cavity) OPS-laser, another (conventional) mirror, serving as an output-coupling mirror is spaced-apart from the OPS-chip, thereby forming a resonant cavity with the mirror-structure of the OPS-structure. The resonant cavity, accordingly, includes the gain-structure of the OPS-chip. The mirror-structure and gain-structure are arranged such that QW layers of the gain-structure are spaced apart by one half-wavelength of the fundamental laser wavelength, and correspond in position with antinodes of a standing-wave of the fundamental laser-radiation in the resonator. The fundamental-wavelength is characteristic of the composition of the material of the QW layers.
Optical pump-radiation (pump-light) preferably from a diode-laser or a diode-laser array (diode-laser bar) is directed into the gain-structure of the OPS-chip and is absorbed by the separator layers of the gain-structure, thereby generating electrical-carriers. The electrical-carriers are trapped in the QW layers of the gain-structure and recombine. Recombination of the electrical-carriers in the QW layers yields electromagnetic radiation of the fundamental-wavelength. This radiation circulates in the resonator and is amplified by the gain-structure thereby generating laser-radiation.
The OPS-laser allows for highly efficient conversion of the low quality, low brightness output of diode-laser bars into a high quality OPS-laser output beam that is both spectrally and spatially near transform limited at power levels of about 10 Watts (W) or greater. Simple thermal area scaling, coupled with semiconductor material improvements, diamond face-cooling of the chip, and improvements in pump light delivering arrangements may permit as much as a 100 W of fundamental output from an OPS laser having only a single OPS-chip. If higher power output is desired than can be provided by a single OPS-chip, however, it will be necessary to provide an OPS-laser including two or more OPS-chips.
U.S. Pat. No. 5,131,002 describes an OPS-laser in which a beam circulating in a laser resonator is caused to make multiple interactions with an OPS-chip having an extended area. This provides the effect of a resonator including multiple individual OPS-chips. This arrangement is depicted schematically in FIG. 1, which, here, is reproduced from a drawing in the '002 patent, with certain elements omitted that are not necessary for understanding a basic principle of the disclosed laser. The laser 10 includes a resonator 12 formed between a highly reflective end mirror 14 and a partially reflective, partially transmissive, output-coupling mirror 16. Circulating radiation in resonator 12 is schematically depicted by long-dashed lines 18.
Included in resonator 12 is an extended OPS-chip 20 including a mirror structure 22 surmounted by a gain-structure 24. An extended, plane reflector 26 is arranged face-to-face with extended OPS-chip 20. Mirrors 14 and 16 of resonator 12 are arranged such that circulating fundamental radiation 18 propagates in a zigzag fashion in the resonator, being reflected alternately from the extended reflector 26 and the mirror structure 22 of the extended OPS-chip. The circulating radiation is incident on extended OPS-chip 20 in areas 28A, 28B, and 28C thereof. Pump light, indicated by short-dashed lines 30, is directed onto areas 28A, 28B, and 28C of the OPS-chip by pump-light sources 32A, 32B and 32C respectively. The pump-light sources are driven by a common power supply 34.
One significant practical problem with the arrangement of laser 10 is that the areas on which the circulating radiation is incident, while originally depicted in the '002 patent as being about equal (presumably for convenience of illustration) would in fact be different. These areas could vary by a factor of four or more (from the largest to the smallest) if the resonator were configured as a stable resonator. This would mean that each pump light delivery system would need to be differently configured in order to avoid inefficiency of pump-light energy coupling into the circulating-radiation. Other practical problems would be encountered in efficiently cooling the extended area of OPS chip 20, and in maintaining an adequate flatness of the chip over the extended area. Changes in flatness during use would cause the actual location of areas 28A-C to change and become misaligned with the location of delivered pump light. This would further decrease optical pumping efficiency.
U.S. Pat. No. 6,097,742 describes a variety of external-cavity OPS-lasers one of which includes two separate OPS-chips. This embodiment is depicted schematically in FIG. 2. Here, the OPS-laser 36 includes OPS-chips 38A and 38B, each of which has a mirror structure 40 surmounted by a gain structure 42. Each OPS-chip is mounted on a dedicated heat sink 44. Laser 36 includes a laser-resonator 37 formed between mirror-structure 40 of OPS-chip 38A and a plane mirror 46. The resonator is folded by mirror-structure 40 of OPS-chip 38A and by a concave mirror 48. Fundamental radiation indicated by arrows F circulates in resonator 37 generally along a longitudinal axis 50 of the resonator as indicated by single arrowheads F. A birefringent filter 52 is included in the resonator for selecting a wavelength of the circulating radiation within the gain-bandwidth of the OPS-gain structures. An optically nonlinear crystal 54 is located adjacent mirror 46 and converts a portion of the fundamental radiation into second-harmonic radiation indicated by double arrowheads 2H. The 2H-radiation is delivered from the resonator through fold mirror 48.
Optical pump light is delivered to each OPS-chip via two optical fibers 56. Each optical fiber transports light from a dedicated diode-laser array (not shown). The pump light is focused onto the OPS-chips by a gradient index lens arrangement 58 as indicated by short-dashed lines P. The pump light is focused onto the chips in a pump-spot size that about matches the size of the beam of circulating radiation at the OPS-chips. Providing optics for focusing light onto the OPS-chips optimizes matching of pump-light energy into the circulating laser beam. However, as the OPS-chips occupy different positions in resonator 37, the mode sizes at the two chips will be different, and the focusing optics (gradient index lenses) would need to be correspondingly adjusted for optimum overall mode-matching.
Without a resonator and pumping arrangement specifically arranged for generating laser radiation from multiple OPS-chips, the efficiency of the resonator can be expected to decrease, as the number of OPS-chips that are included in the resonator is increased. Accordingly, there is a need for a resonator specifically arranged for generating laser radiation from multiple OPS chips.